[DAGCombiner] Reduce load widths of shifted masks

During combining, ReduceLoadWdith is used to combine AND nodes that
mask loads into narrow loads. This patch allows the mask to be a
shifted constant. This results in a narrow load which is then left
shifted to compensate for the new offset.

Differential Revision: https://reviews.llvm.org/D50432

llvm-svn: 340261